In the diagnosis of cardiovascular diseases, cardiac catheterization can be a valuable tool for the cardiologist. During the procedure, the blood pressure can be invasively measured with catheters inserted into the patient's heart or major arteries, and the blood pressure waveform is measured during several heart beats.
However, the blood pressure waveform is also affected by the work of the respiratory system, inducing a low-frequency variation in the waveform. The contraction of the diaphragm compresses and decompresses the lungs and thereby varies the intra-thoracic pressure. Since the pressure measured by the catheter is referred to atmospheric pressure, rather than the actual intra-thoracic pressure, there is a cyclic variation in the observed blood pressure at the respiratory frequency, caused by the patient's respiration.
A further effect of the intra-thoracic pressure variations caused by respiration is the increase of peripheral blood vessel resistance: Increased intra-thoracic pressure will also put pressure on the arteries, which will increase the right ventricular afterload. Afterload is the load the heart must eject blood against during systole. Further, increased pressure on the veins increases the filling of the left atrium, leading to an increased left ventricular output. The net effect of all this will be, that the diastolic pressure in the ventricles will be effected more or less directly by intra-thoracic pressure, while ventricular systolic pressure will be effected by an additional cyclic variation caused by changes in afterload, preload and intra-ventricular dependencies.
In addition, the heart-rate also varies with respiration, a phenomenon called respiratory sinus arrhythmia (RSA). During inspiration, there is an increase in heart-rate, in order to keep cardiac output constant in spite of the increased pressure resistance. Hence, RSA reduces variations in cardiac output caused by respiration, but in return causes large variations in the systolic pressure in the aorta. These variations often appear slightly phase-shifted from the effect of intra-thoracic pressure variations.
The three above described effects influence blood pressure measurements and lead to inaccurate results when calculating diagnostic parameters, such as systolic pressure (SP), beginning of diastolic pressure (BDP), and end diastolic pressure (EDP).
Previous attempts to compensate for these respiratory artefacts have merely used low-pass filters in order to filter out such components oscillating at about the respiratory frequency. Such methods are disclosed for example in the article by S. A. Hoeskel, J. R. C. Jansen, J. A. Blom und J. J. Screuder: “Correction for respiration artefacts in pulmonary blood vessel signals of ventilated patients”, Journal of Clinical Monitoring, 12, pages 397 to 403, 1996.